Torsional fork transducers

ABSTRACT

A torsional fork transducer wherein the two lines of the fork are interconnected at their feet by a piezoelectric crystal operating in the longitudinal mode, the crystal being energized at a rate determined by the natural frequency of the fork to effect torsional vibration of the tines. Oscillators derived from a pick-up electrode on the crystal are applied to the input of an electronic amplifier whose output is fed to a drive electrode on the crystal to sustain the fork in vibration.

United States Patent Dostal I May 30, 1972 [54] TORSIONAL FORKTRANSDUCERS 72 Inventor: Frank 136ml, Elmhurst, NY.

[73] Assignee: Bulova Watch Company, Inc., New York,

[22] Filed: 16, 19 70 211 Appl.No.: 3,349

[52] US. Cl. ....3l0/8.2, 58/23 TF, 3l0/8.7,

2,978,597 4/1961 Harris ..3 10/82 X 3,431,808 3/1969 Oudet etal....3l0/36 UX 3,307,055 2/1967 Schafft ..3 10/83 X PrimaryExaminen-Milton O. Hirshfield Assistant Examiner-B. A. ReynoldsAtt0mey-Michael Ebert ABSTRACT A torsional fork transducer wherein thetwo lines of the fork are interconnected at their feet by apiezoelectric crystal operating in the longitudinal mode, the crystalbeing energized at a rate determined by the natural frequency of thefork to effect torsional vibration of the tines. Oscillators derivedfrom a pick-up electrode on the crystal are applied to the input of anelectronic amplifier whose output is fed to a drive electrode on thecrystal to sustain the fork in vibration.

8 Claims, 7 Drawing Figures TORSIONAL FORK TRANSDUCERS BACKGROUND OF THEINVENTION This invention relates generally to torsional oscillators, andin particular to an oscillator of this type which is exceptionallyefficient and compact, and is adapted to provide a vibratory action ofhigh amplitudes at a constant rate.

Various forms of optical devices are currently in use to chop, modulate,pulse, scan, sweep or otherwise control a light beam or other beams ofradiant energy. Such devices are incorporated in mass spectrometers,bolometers, star trackers, colorimeters, horizon sensors and in variousinstruments which utilize or analyze ion, nuclear, X-ray, laser beams orbeams in the visible, ultra-violet or infrared region.

Existingoptical devices for this purpose usually make use ofmotor-driven discs, drums, mirrors or prisms. Devices using motors arerelatively big and heavy and have large power requirements, particularlyat higher frequencies, thus necessitating extra size and weightprovisions for inverters or similar power supplies. Also in use areelectromechanically-actuated armature devices in which the pivotedarmature is mounted in jeweled bearings. Optical modulators of thesetypes are relatively inefficient and unstable, they are lacking in shockresistance and have other drawbacks which limit their usefulness.

In my co-pcnding applications Ser. Nos. 730,425 filed May 20, 1968, nowUS. Pat. No. 3,532,408, and 814,649 filed Apr..9, 1969, respectively,there is disclosed a torsional oscillator in which an erect torsion rod,anchored at the base, is electromagnetically driven to a point adjacentthe base to cause an optical element attached to the free end of the rodto swing back and forth with an amplitude proportional to the height andQ ofthe rod.

SUMMARY OF INVENTION It is the primary object of this invention toprovide a high Q torsional fork oscillator, which is'exceptionallycompact, efficient, and light weight, and which produces a vibratoryaction of high amplitude at a constant rate determined by the resonancecharacteristics of the oscillator.-

More specifically, it is an object of the invention to provide atorsional fork oscillator which is sustained in vibration bypiezoelectric crystals functioning in the longitudinal mode.

An oscillator in accordance with the invention is adapted to activateoptical devices such as scanners and choppers and for vibratingelectrostatic probes wherein an electrode is alternately exposed to andcut off from an electrostatic field at a periodic rate. I

A significant aspect of the invention is that the torsional forkoscillator is relatively insensitive. to shock forces which may arise inhostile environments, the oscillator having a prolonged operating life,there being no need for lubrication or other care. Operating wear isalmost non-existent and reliability is of a high order. While theinvention will be described in conjunction with an optical scanner, it'is to be understood that it is also useful inany application calling fora vibratory action at a constant rate.

Briefly stated, in a preferred embodiment of the invention, there isprovided a torsional fork having a pair of oscillating tines, whose feetare interconnected by a piezoelectric crystal operating in thelongitudinal mode, the crystal being energized at a rate determined bythe natural frequency of the fork to effect torsional movement of thetines, oscillations derived from a pick-up electrode on the crystalbeing applied to the input of an amplifier whose output is coupled to adrive electrode on the crystal to sustain the torsional fork invibration.

OUTLINE OF" DRAWING FIG. 2 is a side view of the oscillator;

FIG. 3 is a plan view of the oscillator looking at the base thereof; 1

FIG. 4 is a block diagram of the circuit associated with the oscillator;

FIG. 5 is an elevational view of another preferred embodiment of atorsional oscillator in accordance with the invention;

FIG. 6 illustrates the relationship between the torsional element andthe piezoelectric crystals associated therewith; and

FIG. 7, in plan view, shows the compliance member for supporting thetorsional element.

DETAILED DESCRIPTION OF INVENTION Referring now to the drawings, andmore particularly to FIGS. 1 and 3, a torsional fork transducer inaccordance with the invention comprises a pair of parallel torsionalbars 10 and l l, a supporting bar 12 being interposed between thetorsional bars. A strip 13 of flat spring metal is arranged tointerconnect the feet of bars 10, 11 and 12 to hold these bars together.Supporting bar 12 is provided with a stud 14 for attaching the torsionalfork oscillator to a suitable mount.

The basic principles underlying the operation of a torsional fork areset forth in my prior US. Pat. No. 2,877,365 (1959) which shows a tuningfork whose tines are caused to vibrate torsionally substantially alongthe longitudinal axes of the tines, rather than laterally as in aconventional fork. In the torsional fork shown in this patent, the tinesare sustained in vibration by an electromagnetic actuator.

In a tuning fork of traditional design in which the tines vibratelaterally, the restoring force, resulting from the elastic nature of thetines, acts in the transverse direction with respect to the longitudinalaxes of the fork. Hence, when the fork is held with its tines verticallydownward, the restoring force is augmented by the force of gravity,making the fork rate faster. But when the fork is held with its tinesvertically upward, the restoring force is decreased by the gravityfactor, causing the fork rate to slow down. The effect of accelerationon a conventional fork is to multiply the rate error by the number oftimes acceleration exceeds gravity. Thus a standard fork is sensitiveboth to attitude and acceleration.

With a torsional fork, since the torsional vibration takes place aboutthe longitudinal axes of the tines, the restoring force is independentof gravity and acceleration and the fork frequency is free of errorsarising from these factors.

However, while a torsional fork of the type disclosed in my prior patenthas distinct advantages over a conventional fork, the electromagneticactuator disclosed in said patent, which requires the use of crosspieces at the ends of the tines as well as magnets and coils, makes itdifficult to produce a highly compact torsional fork transducer tooperate optical and other devices, in a very restricted space.

In accordance with the present invention, the torsional fork is drivenby a piezoelectric system using crystals of the type which increase ordecrease in length, depending on the potential and the polarity of thevoltage applied thereto. Thus, a given crystal will become longer for agiven polarity and will shorten in length when the polarity is reversed.The crystal material may be of quartz or ceramic material includingbarium titanate, lead zirconate, titanate or other substance possessingpiezoelectric properties and working in the longitudinal mode.

In FIG. 1, two strip-shaped crystals 15 and 16 are used, each crystalbeing provided with suitable electrodes for applying a voltage theretoto effect a dimensional change or for deriving a voltage therefrom inresponse to a dimensional change.

Crystal 15 is attached at its ends tangentially to torsional bars 10 and11 on one side thereof, the midpoint of ,the crystal being attached tosupporting bar 12. Crystal 16 is similarly attached to torsional bars 10and 1 1 and supporting bar 12, but on the other side thereof. Thus, thebars are flanked at their feet by piezoelectric crystal strips.

" sustaining the efiiciency of the system.

Crystal is provided with a narrow electrode 15A running the length ofeach strip to provide a pick-up signal and a broader electrode 158,which is in parallel to electrode 15A but is electrically separatedtherefrom for applying a driving signal to the crystal. Crystal 16 isprovided with an electrode 16A used only fordriving purposes.

As shown schematically in FIG. 4, the pick-up electrode 15A of crystal15 is coupled to the input of an electronic amplifier 17 whose output iscoupled to drive electrode 158 as well as to drive electrode 16A. Inpractice, the amplifier may be a oneor two-stage solid-state amplifier.The fork is sustained invibration at its natural frequency by amplifyingthe pick-up voltage and applying it to the drive crystals in a positivefeedback loop.

Crystals 15 and 16 are oppositely polarized so that as crystal 16expands in length, crystal 15 will concurrently contract,

causing the two torsional bars 10 and 11, tangentially attached thereto,to move angularly about their longitudinal axes in opposite directions.Attached to the free end of bar 10, as best seen in FIGS. 1 and.2, is across rod 18, while attached to the free end of bar 11 is a similarcross rod 19. The moment of rods 18 and 19 and the elasticity of thetorsional bars are factors which determine the vibrating frequency ofthe torsional fork.

Attached to the free ends of torsional bars 10 and 11 are vanes oroptical elements 20 and 21, respectively. in order to 1 obtain a largeswing of these elements, it is essential that the torsional swing at thefree ends of the rods be large. Because the torque is applied by thecrystals at the feet of the rods, mechanical amplification takes place,and it becomes possible to produce a considerable torsional'swing at thefree ends which is well over a hundred times that at the feet. Hence,the

minute changes in dimension of the crystals is sufficient to effect thenecessary swing at the free ends of the rods.

Su port rod 12 acts as a compliance member which pro- .vides couplingbetween torsion bars 10 and 11 and also serves to an extent to absorbslight imperfections in the tuning of the torsion rods. 7

Since the assembly strip 13 is also a frequency-determining element, itcan be chosen to provide a degree of frequency temperature compensationfor the system. For example, if the material of the torsion rods 10 and11 has a negative elastic coefficient, then assembly strip 13 can bemade of a material having a positive elastic coefficient to providecompensation.

In the embodiment shown in FIGS. 5, 6 and 7,- a large vane motion isachieved within a housing of small diameter. in order to reducethediameter of the torsional transducer, instead of apair of paralleltorsion tines, a single torsional rod is used, so supported at itscenter by a compliant mount that the two sections of the rod on eitherside of the mount operate in opposing phase, so that the rod behaves asa straightened-out torsional fork. While this construction results in atransducer which is longer than that shown in FIG. 1, it is advantageouswhere the desideratum is a casing of small diameter rather than onewhich is short in length.

This device is useful in providing a light-weight, small and efficientunit which is suitable for optical devices such as scanners and choppersand for electrostatic probes wherein an electrode is alternativelyexposed to and cut off from an electrostatic field.

The device comprises a torsional shaft 22, supported at its centerbetween frame members 23 and 24 by a mechanical compliance 25 ofcircular shape and made of springy material, thereby effectivelydividing shaft 22 into an upper rod section 22A and a lower rod section2213. A housing 26, attached to frame members 23 and 24, afiordsprotection and shielding. Compliance 25 supports sections 22A and 22B ata nodal point to effect the highest mechanical coupling between the twooppositely vibrating torsional sections, while reducing the couplingbetween the sections and the supporting frame structure. Thus, littlevibration is transmitted to the frame, thereby Crystals 27 and 28 areattached tangentially between upper torsional section 22A and frameelements 23 and 24, respectively. Crystals 29 and 30 are similarlyattached tangentially to lower rod section 228. The crystals are similarto those shown in FIG. 3 and operate in the longitudinal mode.Electrodes 28A and 30A on crystals 28 and 30 provide a pick-up voltagefor the input of an amplifier (not shown) whose output is coupled todrive electrodes 28B and 308 on crystals 28 and'30 and to an electrodeon drive crystals 27 and 29, so that the torsional sections aresustained in vibration. The crystals are connected together in multipleand in the correct phase.

Rod sections 22A and 22B in their portions 22C and 22D adjacentcompliant mount 25 are of enlarged diameter to provide amplitudemagnification and impedance transformation. The motion developed at thepoint on the rod sections 22A and 228 where the crystals are attached ismagnified along the length of the sections both by impedancetransformation and by the mechanical Q of the rod. Magnification in theorder to 50 to times is feasible.

The diameters of the'sections 22A and 22B are also enlarged at the freeend portions 22E and 22F to provide an adequate surface to mount thedevices to be vibrated. In the embodiment shown, a vane 31 is attachedto the free end .of rod section 22A. A counterweight 32 to balance thetorsional system is attached to the free end of rod section 228.

The torsional movement of vane31 actsto bring about alternate exposureand blockage of electrode 32 with respect to the electrostatic fieldcreated by plate 33 through an opening in the vane. Similarly, electrode34 is alternately exposed and cut off with respect to an electrostaticfield charge on plate 35 through an opening in the vane. These vaneopenings and electrodes afford endand side-looking electrostatic probes.The voltages induced in electrodes 32 and'34 are alternating incharacter and, when suitably amplified, provide an indication of thepotential on surfaces 33 and 35.

Similarly, if light detectors are substituted for the elec trodes, andlight sources for the electrostatic charge surfaces, a periodic lightsignal is generated by the vibratingvane. if a mirror is attached to thefree endof rod section 22A, a light scanner is created thereby.

The advantage ofthis system of oppositely vibrating masses is that thereactive forces are self-opposing so that the amplitude of vibration isunaffected by the manner in which the unit is held, ranging from afloating systemin a no-G environment to a hard mount, to a large moment.Nevertheless in practice, one may simplify the torsional transducerstructure by doing away with the lower section of rod and its associatedcrystals, and replacing the compliance, with a hard mount, while addingadequate moment to the housing. If the moment of the housing is madeseveral thousand times the moment of the vane or mirror being vibrated,a satisfactory operation is obtainable. This is analagous to the ratioof the moment of a watch movement to the moment of its balance wheel.

While there have been shown preferred embodiments of the invention, itis to be understood that many changes and modifications may be madetherein without department from the essential spirit of the invention.

What I claim is:

1. A torsional transducer comprising a. a torsional rod, and I Y b.means to sustain said rod in vibration at its natural frequency, saidmeans including a piezoelectric crystal operating in the longitudinalmode, one end of said crystal being tangentially attached to said rod,the other end being attached to a fixed support, said crystal having adrive electrode and a pick-up electrode, said pick-up electrode beingcoupled to the input of an electronic amb. means attaching said rod to afixed mount,

0. and means to sustain said tines in torsional vibration, said meansincluding a piezoelectric crystal operating in the longitudinal mode,the ends of said crystal being tangentially attached to said tines onone side at the feet thereof, said crystal having a drive electrode anda pick-up electrode, said pick-up electrode being coupled to the inputof an electronic amplifier whose output is coupled to said driveelectrode to provide a positive feedback loop.

3. A transducer as set forth in claim 2, wherein said strip is ofresilient material.

4. A transducer as set forth in claim 3, wherein said strip has apredetermined temperature coefficient which is adapted to effectfrequency compensation with respect to the temperature coefficient ofsaid tines.

5. A transducer as set forth in claim 2, further including a secondcrystal attached tangentially to the feet of said tines on the oppositeside thereof, said second crystal having a drive electrode coupled tothe output of said amplifier, said second crystal operating in thelongitudinal mode in phase opposition to the first crystal.

tical elements secured to the free ends of the tines.

7. A transducer as set forth in claim 2, further including 5 crosspieces attached to the free ends of the tines.

8. A torsional fork transducer comprising a an elongated shaft supportedat its center by a compliant mount, the upper section of the shaftconstituting a first torsional rod and the lower section a secondtorsional rod vibrating in phase opposition to the first rod,

b. a pair of frame elements disposed on either side of the shaft andattached to said mount, and

c. means to sustain said first and second rods in torsional vibration,said means including a crystal operating in the longitudinal mode andattached at its ends between a rod section and one of said frameelements, said crystal having a drive electrode and a pick-up electrode,said pickup electrode being coupled to the input of an electronicamplifier whose output is coupled to said drive electrode.

1. A torsional transducer comprising a. a torsional rod, and b. means tosustain said rod in vibration at its natural frequency, said meansincluding a piezoelectric crystal operating in the longitudinal mode,one end of said crystal being tangentially attached to said rod, theother end being attached to a fixed support, said crystal having a driveelectrode and a pick-up electrode, said pick-up electrode being coupledto the input of an electronic amplifier whose output is coupled to saiddrive electrode to provide a positive feedback loop.
 2. A torsional forktransducer comprising a. a pair of torsional tines in parallel relation,a supporting rod at an intermediate position between said tines and astrip interconnecting said tines and said rod at the feet thereof, b.means attaching said rod to a fixed mount, c. and means to sustain saidtines in torsional vibration, said means including a piezoelectriccrystal operating in the longitudinal mode, the ends of said crystalbeing tangentially attached to said tines on one side at the feetthereof, said crystal having a drive electrode and a pick-up electrode,said pick-up electrode being coupled to the input of an electronicamplifier whose output is coupled to said drive electrode to provide apositive feedback loop.
 3. A transducer as set forth in claim 2, whereinsaid strip is of resilient material.
 4. A transducer as set forth inclaim 3, wherein said strip has a predetermined temperature coefficientwhich is adapted to effect frequency compensation with respect to thetemperature coefficient of said tines.
 5. A transducer as set forth inclaim 2, further including a second crystal attached tangentially to thefeet of said tines on the opposite side thereof, said second crystalhaving a drive electrode coupled to the output of said amplifier, saidsecond crystal operating in the longitudinal mode in phase opposition tothe first crystal.
 6. A transducer as set forth in claim 2, furtherincluding optical elements secured to the free ends of the tines.
 7. Atransducer as set forth in claim 2, further including cross piecesattached to the free ends of the tines.
 8. A torsional fork transducercomprising a. an elongated shaft supported at its center by a compliantmount, the upper section of the shaft constituting a first torsional rodand the lower section a second torsional rod vibrating in phaseopposition to the first rod, b. a pair of frame elements disposed oneither side of the shaft and attached to said mount, and c. means tosustain said first and second rods in torsional vibration, said meansincluding a crystal operating in the longitudinal mode and attached atits ends between a rod section and one of said frame elements, saidcrystal having a drive electrode and a pick-up electrode, said pick-upelectrode being coupled to the input of an electronic amplifier whoseoutput is coupled to said drive electrode.